Catalytic processes for the conversion of hydrocarbons using platinum group metals and a catalyst support are well known and extensively used. One such process is reforming. Invariably the catalysts used in these processes become deactivated for one or more reasons. Where the accumulation of coke deposits causes the deactivation, reconditioning of the catalyst to remove coke deposits restores the activity of the catalyst. Coke is normally removed from catalyst by contact of the coke containing catalyst at high temperature with an oxygen-containing gas to combust and remove the coke in a regeneration process. These processes can be carried out in-situ or the catalyst may be removed from a vessel in which the hydrocarbon conversion takes place and transported to a separate regeneration zone for coke removal. Arrangements for continuously or semi-continuously removing catalyst particles from a reaction zone and for coke removal in a regeneration zone are well known.
In order to combust coke in a typical regeneration zone, a recycle gas is continuously circulated to the burn zone of a regeneration section and a flue gas containing by-products of a coke combustion, oxygen and water is continually withdrawn. Coke combustion is controlled by recycling a low oxygen concentration gas into contact with the coke-containing catalyst particles. The flue gas/recycle gas is continuously circulated through the catalyst particles in a recycle gas loop. A small stream of combustion gas is added to the recycle gas to replace oxygen consumed in the combustion of coke and a small amount of flue gas is vented off to allow for the addition of the combustion gas. The steady addition of combustion gas and the venting of flue gas establishes a steady state condition that produces a nearly constant concentration of water and oxygen in the recycle gas.
After the burn zone, the metal-containing catalyst particles drop to a subadjacent halogenation zone. Chlorine or other halogen-containing gas circulates through the halogenation zone in a halogenation loop. Contact of the catalyst with the halogenation gas redisperses platinum group metal on the catalyst particles. The halogen gas added to the halogenation loop sometimes enters the loop in admixture with air or other oxygen-containing gas.
From the halogenation zone catalyst particles descend into a subadjacent drying zone. A heated gas contacts the catalyst particles and drives moisture from the catalyst. Typically, air or an oxygen-containing gas enters the drying zone as the drying medium and passes upward through the halogenation zone to the burn zone to provide combustion gas.
The three different zones provide three potential places for the introduction of air or an oxygen-containing gas into the regeneration system. These three locations are often referred to as upper, middle and lower air and correspond to the relative positions of the upper burn zone, middle halogenation zone, and lower drying zone.
In continuous or semi-continuous regeneration process, coke laden particles are at least periodically added and withdrawn from a bed of catalyst in which the coke is combusted. Regions of intense burning that extend through portions of the catalyst bed develop as the coke is combusted.
The regenerator holds catalyst undergoing regeneration in one or more catalyst beds. Catalyst beds usually take on one of two configurations, a radial flow arrangement, or a vertical flow arrangement. In either type of bed, catalyst must fall freely through the bed to transfer catalyst and obtain a continuous or semi-continuous regeneration. Radial or vertical gas flow through the bed can interfere with the free movement of catalyst particles and hinder the transfer of particles through the bed during the regeneration process.
Experience has shown that horizontal flow of reactants, in particular, through a radial bed of catalyst can interfere with the gravity flow removal of catalyst particles. This phenomenon is usually referred to as hang-up or pinning and it imposes a constraint on horizontal flow reactor designs. Catalyst pinning occurs when the frictional forces between catalyst pills that resist the downward movement of the catalyst pills are greater than the gravitational forces acting to pull the catalyst pills downward. The frictional forces occur when the horizontal flow vapor passes through the catalyst bed. When pinning occurs, it traps catalyst particles against the outlet screen of the reactor bed and prevents the downward movement of the pinned catalyst particles. In a simple straight reactor bed, or an annular bed with an inward radial flow of vapors, pinning progresses from the face of the outlet screen and as the vapor flow through the reactor bed increases, it proceeds out to the outer surface of the bed at which point the bed is described as being 100% pinned. Once pinning has progressed to the outermost portion of the catalyst bed, a second phenomenon called void blowing begins. Void blowing describes the movement of the catalyst bed away from its outer boundary by the forces from the horizontal flow of vapor and the creation of a void between the inlet screen and the outer catalyst boundary. The existence of this void can allow catalyst particles to blow around or churn and create catalyst fines. Void blowing can also occur in an annular catalyst bed when vapor flows radially outward through the bed. With radially outward flow, void blowing occurs when the frictional forces between the catalyst pills are greater than the gravitational forces, or in other words, at about the same time as pinning would occur with a radially inward flow. Therefore, high vapor flow can cause void blowing in any type of radial or horizontal flow bed.
The production of fines can pose a number of problems in a continuous moving bed design. The presence of catalyst fines increases the pressure drop across the catalyst bed thereby further contributing to the pinning and void blowing problems. Catalyst fines can also accumulate in the narrow openings of the screen surfaces used to retain the catalyst particles thereby plugging these surfaces and requiring a shut-down of the equipment to remove catalyst fines. Catalyst fines are usually more abrasive than the larger catalyst particles and thereby contribute to greater erosion of the process equipment. Finally, the catalyst in many of these hydrocarbon conversion processes is a valuable commodity and the generation, and removal of catalyst fines imposes a direct catalyst cost on the operation of the system. Further discussion of catalyst fines and the problems imposed thereby can be found in U.S. Pat. No. 3,825,116 which also describes an apparatus and method for fines removal.
Where possible, horizontal or radial flow reactors are designed and operated to avoid process conditions that will lead to pinning and void blowing. This is true in the case of moving bed and non-moving bed designs. Apparatus and methods of operation for avoiding or overcoming pinning and void blowing problems are shown in U.S. Pat. No. 4,135,886, 4,141,690 and 4,250,018.
In vertical or axial flow beds, the upward flow of gases therethrough can also pose problems of catalyst hang-up and fluidization. As gas passes upwardly through a bed of particles at low velocity, it migrates through the particles without changing the density of the catalyst bed. As velocity increases, the flow of gas creates drag forces that lift the particles. When these drag forces exceed the weight of the catalyst, frictional forces between the particles drop to zero and the bed approaches a fluidized state. With continued increases in gas velocity, the particles experience lift that tends to transport the particles upwardly out of the bed. Some upward gas flow through the bed is useful since it promotes the movement of catalyst particles by reducing interparticle friction so that the particles flow more as a fluid. However, increasing upward drag forces can suspend catalyst particles and prevent downward catalyst movement. As a result, the upward gas velocity through an axial bed of catalyst particles must be limited to permit gravity flow removal of the catalyst particles.
Accordingly, the problems of hang-up catalyst pinning and void blowing limit the gas velocity through the catalyst beds in the regeneration process. The combustion of a fixed quantity of coke in the regeneration process burns a proportional quantity of oxygen. Supplying the stoichiometric oxygen requirements for the coke combustion demands the circulation of sufficient oxygen through the catalyst bed. Higher oxygen demands during transitory periods of heavy coke burning require a temporary increase in the oxygen supply to the burn zone of the regeneration section. Oxygen delivery to the bed varies with the gas velocity and oxygen concentration in the gas stream. Since gravity removal of catalyst particles may limit the gas flow velocity, the only ways to increase coke combustion within the catalyst bed is to increase the oxygen concentration or the size of the bed. Physical dimensions of the regenerator that determine the size of the bed are not easily changed for temporary fluctuations in the process. Charging more air or other oxygen-containing gas to the burn zone increases the available oxygen for coke combustion. Recycle gas normally has a low oxygen concentration of about 0.5 to 2 mol. %. Passing additional oxygen-containing gas or air into the recycle stream quickly raises the oxygen concentration for more coke burning capacity. It is usually preferred to introduce the additional oxygen containing gas into the recycle stream through the drying zone. However, adding the additional oxygen to the drying zone raises the total gas velocity through the drying zone. At some point additional oxygen containing gas cannot be added to the drying zone without raising the gas velocity through the drying zone to a level that causes catalyst lift or hold-up in the drying zone bed. Thus, at maximum gas velocities, additional air cannot be added to the recycle gas through the drying zone. While it is possible to increase the oxygen concentration in the recycle loop without increasing the gas velocity through the drying zone by using oxygen-enriched air, such a method has a disadvantage of requiring additional equipment for the oxygen-enriched air stream.